Hack a Hockney with Gimp

Posted at 11:48am on Friday September 7th 2012

Turn an ordinary digital image into a convincing pop-art masterpiece.

Art aficionados will be familiar with David Hockney, not just for his excellent paintings, but also for his very interesting photography. One technique of his that is often copied is the photo collage, where a single ‘scene’ is made up of dozens, sometimes hundreds, of separate images superimposed onto each other. Without getting all pretentious and carried away, the real visual appeal of these pieces is that by disturbing the detail and elements, it abstracts the form of the scene slightly from the mundane expected reality. Gosh, that does sound pretentious. The important thing to remember is that whilst it is a technique that you can apply to any photograph, it works best with images that have some strong elements of focus in – easily recognisable objects that you will understand even if the edges are made a little wonky or the colours get changed. That’s why the best examples of Hockney’s own work in this area include familiar things such as trees, buildings, roads and quite often text, too.

Here's the 'before' image.

What we are getting at here is that you will have a better result if you choose a good image to start with. Now, when these are done ‘properly’, usually with a film camera, loads of shots are taken and the variations in processing and position, as well as the vagaries of time, weather, changing light and things moving about, produce variations in the individual images. We are going to cheat, because, although we could take 500 images with a digital camera, we are going to fiddle around with things anyway, so we might as well just cheat from the start and choose a single image to work with.
We will start by setting up the image and the layers that we will use for splitting the image into bits and reassembling them. Then we will go through the various steps you might want to take for ‘adjusting’ the individual images; and, finally, we will put it all back together again.

1. Making layers

Once you’ve loaded your background image, add an alpha channel to it, as they don’t have one by default (Layer>Transparency>Add Alpha Channel). The trick is to use lots of layers. As we have an image to begin with, open the Layers window (Control+L). Right-click on the existing layer and choose Duplicate Layer from the pop-up menu. This creates a copy and selects it. Use the Selection tool to define
a rectangular area. Press Ctrl+I to invert the selection and Ctrl+X to clear everything else on the layer. We can now play with these pieces.

2. Change the curves

Changing the contrast and brightness of the image using the eponymous tools is a bit hit and miss. It’s a much better idea to use either Colour>Levels (which at least gives a more fine-grained control) or better still, Colour>Curves. Just drag the curve into the position you want – dragging it above the centre line makes those parts of the image brighter, and below makes them darker. Obviously, don’t adjust them all exactly the same – some parts may benefit from more contrast, others might be better being more flat.

3. Mess with the colour

As well as playing with the brightness and contrast of the image, you can mess with the colour values. This can be particularly effective when trying to highlight certain objects – as with this image, where you can see that we have increased the bluish tones of the car in many places. You can do this using Colours>Curves and adjusting the required channels individually. You should also experiment with adjusting Colours>Hue-Saturation and increasing the saturation level – don’t overdo it or the image will look fake and over-processed.

4. Reposition everything

Part of the idea behind making a collage like this is to break up the outline and shape of the objects in the scene – it is the act of removing the ‘image’ of the object and replacing it with an ‘impression’. Many of the adjustments here can help with that, but repositioning the pieces is by far the most effective. As with most things, it pays to be a bit subtle about it, but you should probably try to reposition everything at least a little bit. The Repositioning tool from the toolbar is obviously the one for this job.

5. Scale it

Scaling sections of the image not only helps a bit with making all the pieces seem as if they are not from the same picture, but also makes it possible to highlight certain features of the image to make more of a caricature of the objects in the scene. You will see this a lot in original Hockney works, and it is easy to do. With the area selected, select the Scale tool and, while holding down the Ctrl key, drag out
a corner. Holding the key down constrains the proportions so it doesn’t get all distorted when scaling. You should make sure you reposition the image after doing this.

6. Rotate a piece

Rotating the selected area is something you should try to do a lot. It is these little adjustments which disjoint the image and break up the continuity of the objects, so all in all it’s a good thing. If you have your rectangle selected, it’s better to use the Rotation tool from the toolbar. This will auto-centre on the selection, and you can use the slider to adjust it by the required amount – you will get an on-screen preview. Don’t get too carried away – even a small adjustment of
a few degrees is enough to make a difference – too much risks making the finished image a bit unintelligible.

7. Perspective

The hardest thing to fake when starting with a single image is the impression the images have been taken from a different position. You can do a reasonable approximation of this using the Perspective tool, though, as long as you’re careful not to distort the image so that it looks unnatural. Make sure you have the rectangle area selected, and click on the Perspective tool and drag out the corners. It works best if you drag only one side out – you don’t want to twist the image too much. You should start with an area bigger than normal, as you will need to cut it out again afterwards to turn it back into a rectangle.

8. Borders

On some Hockney-esque images, people choose to add a border to each separate ‘piece’ of the image. From experience, it is safe to say that this technique can work sometimes, but quite often it gets a bit out of hand. It only really works well if all of your rectangles are of similar sizes. In any case, it is quite easy to add a border. While you still have the rectangle selection, choose Selection>Border and enter a suitable value for the pixel width depending on your image.
Then choose Edit>Fill with BG Colour to fill it white (or any other colour you like).

9. Cheat!

You will soon realise that making dozens of layers of tiny rectangles is a time-consuming business. Even if you can manage to bash them out quite quickly, it will still take several hours to complete an image. The solution is to cheat. There is nothing stopping you from using these techniques on several areas at a time (even the rotate if you are careful). Instead of selecting just a single rectangle from the image, select several. Hold down the Shift key and drag out more rectangles. Make sure they are in areas of the image which are similar in terms of the treatment you want (eg, in tone or colour), and of course, they shouldn’t be too close together physically, as this will spoil the effect.

10. Cheat more!

Even if you use the previous cheat and process on several areas of the image at a time, you may still find that it takes a long time to complete a whole image. Just think of the poor people who do this sort of thing with real pictures! Anyway, there is another cheat you can use – just keep the original background image in place to fill up any gaps. The more astute of you will realise that this will destroy the illusion of the irregular edges, but the even more clever will also realise that you can just cut it out. Use the Selection tool to select the inner area of the image you want to use to fill in gaps – Ctrl+I to invert and Ctrl+X to remove the edges.

11. Save as you go

It is obviously a good idea to save out the image regularly, perhaps in several different versions. Depending on the size of your image, dealing with all those layers can take up a fair amount of memory.
For example, at one point the saved file size for this image was just
a shade under 1GB, which I think you will agree is pretty large. Gimp, and Linux, are very stable, and there is unlikely to be a system crash just because you use a lot of memory, but it can slow things down considerably. One solution is to use the Layer>Merge Down option to flatten bits that you are already happy with.

12. Merging layers

The final thing left to do is to flatten the image. Before you do this, though, you might want to reposition some of your layers. The way layers work means that your image might be a bit back-to-front,
as new layers generated from the background are inserted directly above – so the ones you created first will be on top. Take a moment
to drag them around in the Layer view until you’re happy with the positioning. Once you have finished (save a version first!), you can choose Image>Flatten Image to merge all the layers, and then make any final adjustments.